Hard disk drives (HDDS) in computer products are sensitive to external vibration, either from other devices or from sources outside the system. For example, the effect of external vibration on HDDs is to disturb the tracking of read/write heads on data tracks, to increase the track access time, and to reduce overall data transfer rates. Further, in case of a significant vibration, read performance and, especially, write performance of HDDs can be severely reduced. If the vibration becomes large enough, writing to HDD may be prevented and it may cause a failure of the system.
As computers, such as computer servers and storage systems, become more and more dense, the number of internal vibration sources has increased. Because these dense systems require more cooling, the energy to be put into internal cooling fans has increased in terms of the number of fans, total power of motor fan drive, and rotational speed of fans. In particular, fan speeds for the small systems have more than doubled in recent years. In the past, the only significant source of HDD vibration disturbance was other HDDs in the system. On the other hand, now, cooling fans have become the major cause of HDD performance problems in many systems.
The vibration sources within fans are many and variable. The most common cause of fan-induced HDD vibration sensitivity is fan torque ripple off and on the axis of fan rotation at frequencies above 300 Hz and especially above 500 Hz to 1 kHz or higher. Despite a common trend toward increasing vibration energy at increasing frequencies, different fan designs in consideration for a single system product may have very different spectrum characteristics. Also, the impact of even the same fan at different speeds is affected by fan motor characteristics, chassis structural response, and HDD servo response.
The impact of fans on HDD performance can be serious. Without adequate fan characterization methods, the impact is quite unpredictable. No commercial methods currently exist for the measurement of fan dynamics, especially for the vibration components on top of steady fan torque or for the components off the motor axis.